1. Field of the Invention
The present invention relates to antennas and more particularly to antennas of the type called parabolic reflector antennas intended to be used on board a telecommunications satellite, and a method for manufacturing these antennas.
Technicians skilled in this matter know that, in the field of satellite links, several types of antennas may be used, particularly:
approximately isotropic antennas which require no particular orientation of the satellite, but which waste the transmitting power, PA1 dipoles, whose antenna pattern has a maximum in the plane perpendicular to its axis, which plane must be oriented towards the Earth by stabilizing the satellite by rotation about an axis parallel to the dipole, PA1 horn antennas with overall coverage, whose antenna pattern coincides with a cone having a flare angle of 17.degree., but which must remain pointed towards the Earth, and PA1 parabolic reflector antennas covering a zone limited to a Continent or a region, even a country, and whose gain increases inversely with the flare angle of the beam. PA1 Coating the surface with a film which serves as mask and in which the structure of the grid has been previously cut out, PA1 Vacuum deposition of an appropriate metal in the vapor state, PA1 Removal of the mask so as to leave the metallized strips in position; PA1 Application of a metal grid using a photochemical method on the adhesive ribbon strips made from "Kapton"; in this case, the arrangement of these strips, thus metallized, in parallel strips on a parabolic surface is obtained by cutting the ribbon up into small pieces which are then stuck together; PA1 said grid to be disposed on a parabolic surface is obtained by developing this latter on a flat sheet and cutting it out from this sheet: it should be noted that on this sheet said strips--which are parallel in the parabolic surface--are no longer parallel to each other (cf. FIG. 4, as well as the patent US-4 001 836). The grid thus developed is transferred by a photochemical method to an adhesive strip made from "Kapton". Then, this adhesive strip is bonded in sections on the surface of the reflector so as to form the grid or parallel elements typical of PSS reflectors; PA1 incorporation of metal wires forming the desired grid in a laminated structure. PA1 levelling out or smoothing the irregularities of the surface of said support by application of an appropriate levelling means, such as a lacquer or a polymer film or similar, PA1 total metallization of this surface thus levelled, by metal deposition under a vacuum, PA1 forming of a mask (or screen) having orifices corresponding to the elements of the desired grid, PA1 interpositioning of this mask between an appropriate laser source and a focusing lens, PA1 focusing of the laser beam emitted by this source on the metallized surface of the reflector while following the contours of the orifices formed in the mask, PA1 scanning of the surface of the reflector by means of the laser beam for cutting out said grid therein, PA1 compensation of the variations of distance between the surface of the parabolic reflector and the laser source, and thus the focusing optics of the laser beam emitted thereby, during scanning of this surface.
2. Description of the Prior Art
Now, among the parabolic reflector antennas, the two following types are used, antennas (or reflectors) with a surface sensitive to the polarization and antennas (or reflectors) with a surface sensitive to the frequency, (known as "polarization-sensitive surfaces", PSS, and "frequency-sensitive surfaces" FSS, respectively: see the Antenna Engineering Handbook by Johnson/Jasik-Mc Graw Hill). In so far as polarization sensitive reflectors are concerned, they are used on board telecommunications satellites for providing frequency re-use in the case of double linear polarization.
The surface sensitive to the polarization of the signals is formed by a plurality of parallel linear reflectors, consisting of aluminum, copper or other metal strips, and defining a grid g on the surface of the reflector (also grid reflector: cf. also the reference Rp in FIG. 1a accompanying the present description), which linear reflectors reflect a polarization and transmit the perpendicular polarization.
The support structure for said grid is formed by a honeycomb core n sandwiched between two "Kevlar" sheets Kv1, and Kv2 (cf. FIG. 1b).
In so far as the frequency sensitive reflectors are concerned, they are used for the simultaneous generation of different beams in more than one frequency band using the same antenna (in other words, these reflectors transmit the signals of one frequency band and reflect the signals of another frequency band).
The surface sensitive to the frequency of the signals (or dichroic surface) is formed by a plurality of separate resonating elements, but disposed side by side and defining on the surface of the reflectors a double grid (we speak of double grid reflector) of resonating elements which are required for separating frequencies whose ratio is between 1.5/1 and 2/1. Different types of elements are used, such as rings, (cf. FIG. 2a), crossed dipoles (cf. FIG. 2b), square loops (cf. FIG. 2c), Jerusalem crosses (cf. FIG. 2d) and tripoles (not shown, but each defined by a star with three arms separated by 120.degree. from each other). The mechanical constraints and those due to the environment mean that the double grid resonating elements must be embedded in laminated dielectric structures of the honeycomb type made from "Kevlar" (cf. FIG. 5).
Now, the construction of said polarization and frequency sensitive surfaces is very delicate. In fact:
in so far as the spacing between the axes of the two adjacent metal strips of the PSS reflectors and the width of these strips are concerned, these parameters must be optimized for minimizing the insertion losses and maximizing the suppression of cross polarization: for example, for the frequency of 12 GHz, the width of the metal strips and the spacing between the axes of two adjacent strips are of the order of 0.1 mm and 0.25 mm, respectively; furthermore, the thickness of the metal strips must be kept at a minimum value for reducing the thermal distortion of the reflectors due to the different thermal expansion coefficients of the laminated support structure and of the grid,
in so far as the dimensioning of the resonating elements of an FSS (or dichroic) reflector are concerned, this is critical for optimizing the electric behavior: for example, elements must be formed whose width is about 25 mm, which is very delicate; moreover, it is also necessary in this case to make the thickness of the metal resonating elements minimum, so as to reduce the thermal distortion.
The critical aspect of the dimensioning of the dichroic reflectors also exists in connection with a particular category of this type of reflector, which is represented by the thermal shields transparent to radio frequencies. These shields are intended to reflect the infrared radiation and to filter as much as possible the solar rays so as to limit the temperature rise. They are formed from a "Kapton" dielectric support coated with aluminum on the inner side, the aluminum coating being etched in accordance with the pattern providing efficient transparency to the radiofrequencies. Now, so that the transmission losses are very small, it is vital for the dielectric support and the metal elements etched thereon to have, respectively, the smallest possible thickness and dimensions: a typical value for the square elements (see FIG. 3) is 90 .mu. for the corresponding side, with about 7 .mu.m between two adjacent elements.
In so far as the techniques are concerned for manufacturing PSS reflectors, the four methods mentioned below are used for applying the above mentioned metal grid to the "Kevlar" sheet of a reflector of this type:
First method
Second method
Third method
Fourth method
However, all these methods are time consuming due to complying with manufacturing tolerances, both from the mechanical and the electrical points of view. Furthermore, bonding the grid in the form of a prefabricated ribbon adds to these problems the one of stability of the junction lines in an environment as severe as the space environment and, consequently, those of increasing the insertion losses and of degradation of the performance of the antenna.
The typical drawbacks of the four above mentioned methods are the following:
in so far as the first method is concerned, cutting out the mask limits the configuration which can be given to the grid, and in particular the optimization of the spacing between the axes of the strips and the width of these latter which is required for adaptation to higher frequencies,
in so far as the second and third methods are concerned, bonding the adjacent strips to a non-flat surface is critical for keeping the ohmic losses to low values, which involves the use of optical alignment control means which are very time consuming.
In so far as the fourth method is concerned, it does not satisfy the requirement of good electric performance because of the lower limits existing for the diameter of the metal wires and the dimensional parameters of the grid.
As for the technique for manufacturing FSS reflectors, it should be stated that it consists in disposing two layers of "Kapton" Kp1 and Kp2--in which metal (dichroic) dipoles D have been previously embedded--between "Kevlar" layers Kv and honeycomb layers (cf. FIGS. 5a and 5b).
However, since the thermal expansion coefficient of metallized layers differs greatly with respect to that of the "Kapton" substrate, a relatively thick adhesive layer must be used, which adds to the problem of manufacturing tolerances, as mentioned above, the problem of stability of the composite structure.
Furthermore, in so far as the manufacture of said thermal screens transparent to radio frequencies is concerned, the metal squares, illustrated in FIG. 3, on a "Kapton" sheet Kp are at present obtained by photo-etching.
However, the limitations inherent in present photo-etching methods prevent the construction of one piece thermoscreens of large dimensions, which means that they must be manufactured by assembling component elements of small dimension and, consequently, by increasing manufacturing time and costs.
The patent FR-2 152 671 describes a method of producing a dielectric material layer charged with an electroconducting material, particularly for covering radomes, in which the latter is deposited on the former in an appropriate configuration defining identical zones of electroconducting material. This configuration is obtained either by selectively etching an electroconducting material layer applied to the dielectric surface or by applying to this latter a screen having apertures whose contours define a zone corresponding to said electroconducting zones, and by applying the electroconducting material through these apertures. However, apart from the use of a screen of said type, the method described in patent FR-2 152 671 has nothing to do with the method of the invention.
The purpose of the present invention is to provide a method of manufacturing a parabolic reflector antenna, in particular of the type having a polarization sensitive surface (PSS) or frequency sensitive surface (FSS), which method answers better the requirements of practice than previously known methods aimed at obtaining the same results, particularly in that the manufacturing tolerances obtained ensure optimum performances for the reflectors thus produced, and in that the manufacturing time is considerably reduced.